Page 191 - Geology of Carbonate Reservoirs

172 DIAGENETIC CARBONATE RESERVOIRS

geometry. Wireline logs, seismic profiles, and even borehole images do not provide
enough information to identify and classify porosity; therefore it is absolutely neces-
sary to examine rock samples, preferably full - diameter cores. Although it is possible
to identify and classify porosity in samples from a single well, it is not possible to
predict the size and shape of the three - dimensional porous reservoir without addi-

tional information about how the one well fits in the field - scale stratigraphic and

structural architecture. This information is usually obtained as additional wells are
drilled. Predicting the size and shape of the reservoir body is the most basic task of
the geoscientists and engineers who must decide where to drill the next well and

how to develop a field in the most cost - effective manner. Identification and classifi -
cation of genetic pore types is the first step in determining the geological origin of

a reservoir, predicting its spatial dimensions, and, ultimately, in delineating individ-

ual flow units, baffles, and barriers.
The following checklist includes procedures for identifying and exploiting diage-
netic reservoirs.

1. Classify the genetic pore types — purely diagenetic, hybrid type 1A (deposi-

tional attributes dominant), and hybrid type 1B (diagenetic attributes domi-
nant) — to determine which has the most influence on reservoir quality. Identify

the type of diagenesis — cementation, compaction, dissolution, replacement, or

recrystallization — associated with the pore types in flow units, baffl es, and
barriers.

2. Identify the diagenetic environment or environments in which pore alteration
took place, how many episodes of change took place, and in what order of
occurrence the changes took place. The point is to determine which diagenetic

events caused changes that have the greatest influence on today ’ s porosity and
permeability, in which environments those changes took place, when during
burial history they took place, and how to use that information to predict the

spatial distribution of flow units, baffles, and barriers.

3. Search for evidence of paleoaquifers, exposure surfaces, stratigraphic cycles
that may include evaporites, paleosols, or karst features by comparing litho-
logical logs from cores or cuttings with subsurface geological data (structure
and stratigraphy), seismic profiles and seismic attributes, biostratigraphic data,

and geochemical data. If joints or fractures are part of the pore systems, frac-
ture geometry usually occurs in predictable patterns on faults and folds.
4. Diagenetic porosity may correspond to present structure, paleostructure, prox-

imity to unconformities, proximity to facies such as evaporites or lacustrine
deposits, or proximity to fracture systems that were conduits for migrating
fluids. Compare structure maps with interval porosity maps to test for contour

shape similarity. Strong correspondence between structural highs and high
porosity values indicates that structural form has remained basically unchanged
since diagenesis took place. It also suggests that paleotopographic highs were
more affected by diagenesis than paleo - lows.
5. If porosity and present structure maps do not have similar shapes, test for

correspondence between porosity and paleostructure. Make interval isopach
maps of the thinnest interval possible that overlies the reservoir. Isopach thins
indicate antecedent highs on the reservoir surface; thicks indicate lows. If